1. Field of the Invention
This invention relates to a method and apparatus for removing fixed pattern noise from a signal produced by a solid state image sensing array, and more particularly to removing fixed pattern noise of the type manifested as a fixed percentage deviation between the apparent sensitivities of the sensor elements in the array.
2. Discussion Related to the Problem
An image sensor comprised of an array of solid state sensor elements produces a signal composed of a train of pulses. The magnitude of each pulse represents the size of the charge packet produced by the corresponding sensor element in the array. Generally, the signals from such arrays are subject to two types of pattern noise. One type of pattern noise, caused, for example, by dark current in a sensor, results in a charge packet size bearing little or no relation to the actual amount of light that fell on the sensor element. A second type of pattern noise is manifested as a percentage variation in the apparent sensitivity of an element compared to the sensitivities of other elements. Such percentage variations are caused by manufacturing tolerances in the physical dimensions of the elements, or variations in transparency of electrodes from element to element. Apparent sensitivity variations may also result from non-uniform, non-imagewise illumination. Another source of pattern noise of this second type is non-uniformities in a color filter array disposed over the array to render the array sensitive to color information.
In the prior art, fixed pattern noise of the second type noted above has been corrected by making a predetermined percentage change to a signal representing the size of charge packet. See SMPTE Journal, April 1978, Vol. 87, No. 4, "An Experimental Telecine Using a Line Array CCD Sensor" by I. Childs and J. R. Sanders, p. 209-213. In the apparatus employed by Childs and Sanders, the percentage adjustment of a signal is accomplished by premeasuring the output of a sensor under non-imagewise input conditions. A first digital signal representing the logarithm of the magnitude of the charge packet produced by the sensor is generated and stored in memory. During normal operation of the device, i.e. with an imagewise input, the logarithm of the magnitude of the charge packet produced by the sensor is subtracted from the first digital signal using digital circuitry. The signal representing the remainder, after the subtraction, is converted from digital to analog form and when suitably amplified, used as the corrected signal. This process effectively normalizes all of the signal magnitudes to remove any percentage variations in apparent sensitivity from sensor to sensor. However, the digital-to-analog and analog-to-digital conversions and the log and antilog computations involved in this process require complex electronic circuitry. Furthermore, the computations and conversions introduce rounding errors that contribute to the noise level of the final signal. Also, operations on the signal are performed after the charge packet produced by the sensor has been read out of the sensor, by which time noise of other types such as clock feedthrough has been introduced into the signal.
Thus, the problem to be solved is one of providing a relatively simple and efficient method and apparatus for correcting fixed pattern noise of the percentage variation type in the output of a solid state sensor.